1. Field of the Invention
This invention relates to an improved process combination for the conversion of hydrocarbons, and more specifically for the selective upgrading of naphtha by a combination of selective olefin formation and aromatization.
2. General Background
The widespread removal of lead antiknock additive from gasoline and the rising fuel-quality demands of high-performance internal-combustion engines have compelled petroleum refiners to install new and modified processes for increased "octane," or knock resistance, in the gasoline pool. Refiners have relied on a variety of options to upgrade the gasoline pool, including higher-severity catalytic reforming, higher FCC (fluid catalytic cracking) gasoline octane, increased alkylation of paraffins and olefins, isomerization of butanes and light naphtha and the use of oxygenated compounds.
Catalytic reforming is a major focus, as this process generally supplies 30-40% or more of the gasoline pool. Increased reforming severity to obtain higher-octane reformate generally results in higher production of fuel-value light gases and a lower yield of the desired C.sub.5 + reformate. Since this yield effect is magnified at higher reforming severity, workers in the art are faced with an increasingly difficult task of improving reforming catalysts and processes in order to maintain the yield of gasoline-range product.
One focus has been on the relative importance and sequence of the principal reforming reactions, e.g., dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins to aromatics, isomerization of paraffins and naphthenes, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. High yield of desired gasoline-range products are favored by the dehydrogenatlon, dehydrocyclization and isomerization reactions. The dual-function nature of reforming catalysts facilitates ready conversion of alkylcyclopentanes as well as cyclohexanes through isomerization in conjunction with dehydrogenation. Considering that reforming generally is effected in a series of zones containing catalyst, naphthene conversion to aromatics usually takes place principally in the first catalyst zones while paraffin dehydrocyclization and hydrocracking occurs primarily in subsequent catalyst zones.
The usual sequence of reforming reactions may be addressed advantageously through staging of catalysts containing different metals within a single reforming process unit. U.S. Pat. No. 4,929,333 (Moser et al.) teaches a germanium-containing reforming catalyst ahead of a germanium-free catalyst preferably containing rhenium and also cites other art appropriate to this concept.
Nonacidic zeolitic catalysts are known to be particularly effective for aromatization of paraffins through dehydrocyclization as well as for dehydrogenation of naphthenes. The staging of zeolitic catalysts for selected reactions also is recognized. U.S. Pat. No. 4,645,586 (Buss) teaches reforming using the sequence of a bifunctional catalyst having acid sites and containing a Group Vil metal followed by a nonacidic catalyst containing a large-pore zeolite (preferably L-zeolite) and a Group Vil metal. U.S. Pat. No. 5,037,529 (Dessau et al.) discloses dual-stage reforming the feed in the first stage with a nonacidic medium-pore zeolite containing a dehydrogenation/hydrogenation metal and Sn, In or TI, and converting first-stage effluent in the second stage with an acidic zeolite catalyst having a constraint index of 1-12.